In general, as solar cells for converting solar energy into electricity, there have been proposed various solar cells such as silicon-based, compound-based, and organic-based solar cells. It is considered that since the silicon-based solar cells use, as its material, silicon which is present in a large amount as an earth resource, the problem of resource exhaustion or the like does not arise as compared with the other compound-based and organic-based solar cells.
The silicon-based solar cells can be classified into the monocrystalline type, the polycrystalline type, and the amorphous type. Among these silicon-based solar cells, the monocrystalline type and polycrystalline type solar cells are of the bulk type while the amorphous type solar cell is of the thin film type. Herein, it is necessary to form a film having a thickness of about 100 μm for the bulk type silicon solar cell while it is sufficient to form a film having a thickness of about 0.5 μm for the thin film type silicon solar cell.
Recently, among the above-mentioned various solar cells, the bulk-type silicon solar cells tend to be widely used because the energy conversion efficiency is relatively high and further the manufacturing cost is relatively low. However, with the rapid increase in demand for the bulk-type silicon solar cells, a large amount of monocrystalline and polycrystalline silicon might be needed as materials and result in a serious increase in cost. This also might make it difficult to acquire the materials.
In the meanwhile, it has been proposed to use the solar energy as an alternative energy that substitutes for the thermal or hydraulic power. In order to use the solar energy as the alternative energy for substituting the thermal or hydraulic power, there is required a solar cell facility having a large area on the order of 1 km2 to supply high power instead of the thermal or hydraulic power.
In the case of the monocrystalline type or polycrystalline type silicon solar cell, it is necessary to form a thick monocrystalline or polycrystalline silicon film, and therefore, it is very difficult to produce the solar cell facility with a large area and high power in terms of both cost and resource as also described above.
In contrast, in the case of the amorphous type silicon solar cell, the thickness of an amorphous silicon film can be 1/100 or less as compared with the monocrystalline type or polycrystalline type silicon solar cell, and therefore, it is suitable for actually manufacturing, at a low cost, the solar cell facility with a large area and high power.
However, it is pointed out that the amorphous type silicon solar cell is disadvantageous in that the energy conversion efficiency is about 6% to 7% and is extremely low as compared with the monocrystalline type or polycrystalline type silicon solar cell having an energy conversion efficiency of about 20%. In addition, it is also pointed out that the energy conversion efficiency of the amorphous type silicon solar cell decreases with an increase of the area.
Patent Document 1 discloses forming a transparent electrode having an oblique section on a substrate in order to improve the performance of a thin film type solar cell and using one of ZnO, SnO2, and ITO as the transparent electrode.
Patent Document 2 discloses an amorphous type silicon solar cell having SnO2 or ZnO as a transparent electrode layer.